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The test uses a thick-walled sample tube, with an outside diameter of 50mm and an inside diameter of 35mm, and a length of around 650mm. This is driven into the ground at the bottom of a borehole by blows from a slide hammer with a weight of 63.5kg (140 lb) falling through a distance of 760mm (30 in). The sample tube is driven 150mm into the ground and then the number of blows needed for the tube to penetrate each 150mm (6in) up to a depth of 450mm (18in) is recorded. The sum of the number of blows required for the second and third 6 in. of penetration is termed the "standard penetration resistance" or the "N-value". In cases where 50 blows are insufficient to advance it through a 150mm (6 in) interval the penetration after 50 blows is recorded. The blow count provides an indication of the density of the ground, and it is used in many empirical geotechnical engineering formulae. The main purpose of the test is to provide an indication of the relative density of granular deposits, such as sands and gravels from which it is virtually impossible to obtain undisturbed samples. The great merit of the test, and the main reason for its widespread use is that it is simple and inexpensive. The soil strength parameters which can be inferred are approximate, but may give a useful guide in ground conditions where it may not be possible to obtain borehole samples of adequate quality like gravels, sands, silts, clay containing sand or gravel and weak rock. In conditions where the quality of the undisturbed sample is suspect, e.g. very silty or very sandy clays, or hard clays, it is often advantageous to alternate the sampling with standard penetration tests to check the strength. If the samples are found to be unacceptably disturbed, it may be necessary to use a different method for measuring strength like the plate test. When the test is carried out in granular soils below groundwater level, the soil may become loosened. In certain circumstances, it can be useful to continue driving the sampler beyond the distance specified, adding further drilling rods as necessary. Although this is not a standard penetration test, and should not be regarded as such, it may at least give an indication as to whether the deposit is really as loose as the standard test may indicate. The usefulness of SPT results depends on the soil type, with fine-grained sands giving the most useful results, with coarser sands and silty sands giving reasonably useful results, and clays and gravelly soils yielding results which may be very poorly representative of the true soil conditions. Soils in arid areas, such as the Western United States, may exhibit natural cementation. This condition will often increase the standard penetration value. The SPT is used to provide results for empirical determination of a sand layer's susceptibility to earthquake liquefaction, based on research performed by Harry Seed, T. Leslie Youd, and others. Correlation with Soil Mechanical Properties Despite its many flaws, it is usual practice to correlate SPT results with soil properties relevant for geotechnical engineering design. The reason being that SPT results are often the only test results available, therefore the use of direct correlations has become common practice in many countries. Different correlations are proposed for granular and cohesive soils. University of Missouri - Rolla Class notes on the SPT. Topics in geotechnical engineering Clay Silt Sand Gravel Peat Loam Hydraulic conductivity Water content Void ratio Bulk density Thixotropy Reynolds' dilatancy Angle of repose Cohesion Porosity Permeability Specific storage Effective stress Pore water pressure Shear strength Overburden pressure Consolidation Soil compaction Soil classification Shear wave Lateral earth pressure Cone penetration test Standard penetration test Exploration geophysics Monitoring well Borehole Atterberg limits California bearing ratio Direct shear test Hydrometer Proctor compaction test R-value Sieve analysis Triaxial shear test Hydraulic conductivity tests Water content tests Crosshole sonic logging Nuclear Densometer Test Bearing capacity Shallow foundation Deep foundation Dynamic load testing Wave equation analysis Mechanically stabilized earth Soil nailing Tieback Gabion Slurry wall Mass wasting Landslide Slope stability analysis Soil liquefaction Response spectrum Seismic hazard Ground-structure interaction Geotextile Geomembranes Geosynthetic clay liner Cellular confinement Instrumentation for Stability Monitoring Deformation monitoring Automated Deformation Monitoring I am an expert from 78wy.com, while we provides the quality product, such as safety tape , China with tray, China Suppliers,and more.
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